The present invention relates to a method of manufacturing a carcass structure for tyres, in particular for two-wheeled vehicles, comprising the steps of: preparing strip-like sections each comprising longitudinal and parallel thread-like elements at least partly coated with at least one layer of raw elastomer material; making at least one carcass ply by laying down and circumferentially distributing said strip-like sections on a toroidal support, each of said strip-like sections extending in a U-shaped configuration around the cross-section outline of the toroidal support, to define two side portions mutually spaced apart in an axial direction, and a crown portion extending at a radially outer position between the side portions; applying annular reinforcing structures to an area close to inner at circumferential edges of said at least one carcass ply.
The invention also relates to a carcass structure for tyres obtainable by the above mentioned method, said carcass structure comprising: at least one carcass ply comprising strip-like sections circumferentially distributed around a geometric rotation axis, and each comprising at least two thread-like elements disposed longitudinally and parallelly of each other and at least partly coated with at least one layer of raw elastomer material, each of said strip-like sections extending in a substantially U-shaped conformation around the cross-section outline of the carcass structure, to define two side portions spaced apart from each other in an axial direction, and a crown portion extending at a radially outer position between the side portions.
Accomplishment of tyres for vehicle wheels involves formation of a carcass structure essentially made up of one or more carcass plies of a substantially toroidal conformation and having their axially opposite side edges in engagement with respective circumferentially-inextensible annular reinforcing elements, usually called xe2x80x9cbead ringsxe2x80x9d.
Applied to the carcass structure, at a circumferentially outer position thereof, is a belt structure comprising one or more belt strips in the form of a closed ring, essentially consisting of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
A tread band currently consisting of a strip of elastomer material of appropriate thickness is applied to the belt structure, at a circumferentially external position thereof.
It is to point out that, to the aims of the present description, by the term xe2x80x9celastomer materialxe2x80x9d it is intended a rubber blend in its entirety, that is the assembly made up of at least one polymer base suitably amalgamated with reinforcing fillers and/or process additives of various types.
Finally, to the opposite sides of the tyre being manufactured a pair of sidewalls is applied, each of them covering a side portion of the tyre included between a so-called shoulder region, located close to the corresponding side edge of the tread band, and a so-called bead located at the corresponding bead ring.
In accordance with traditional production methods, essentially, the above listed tyre components are first made separately from each other, to be then assembled during a tyre-manufacturing step.
It is the Applicant""s feeling that these production methods are so far at least partly unsatisfactory in terms of quality of the finished product, and that they have some critical points as regards accomplishment of the process which is complicated and hardly governable.
For instance, for making the carcass ply or plies to be associated with the bead rings so as to form the carcass structure, it is required that, by an extrusion and/or calendering process, a rubberized fabric comprising longitudinally-disposed continuous textile or metal cords should be first produced. This rubberized fabric is submitted to a transverse-cutting operation to produce sections of predetermined sizes that are subsequently joined together so as to give origin to a continuous ribbon-like semifinished product, having transversely-disposed parallel cords.
This article of manufacture will be then cut into sections the length of which is correlated with the circumferential extension of the carcass to be made.
Production methods have been also proposed which, instead of resorting to the production of semifinished products, make the carcass structure directly during the tyre-manufacturing step.
For example, U.S. Pat. No. 5,362,343 discloses a method and an apparatus forming a carcass ply starting from a single cord previously wound around a reel.
According to the method and apparatus described in the above patent, at each operating cycle of the apparatus the cord drawn from the reel by power-driven idler rollers and maintained taut by a pneumatic tensioning system is cut to size to obtain a section of predetermined length.
The cord section is picked up by a pick-up element mounted on a belt wound over power-driven pulleys to be extended transversely on the outer surface of a toroidal support.
The section ends are then engaged by bending members of the belt type operating on the opposite sides of the toroidal support for radially applying the cord section to the toroidal support by means of slide elements acting like fingers along the side portions of the cord section.
Repetition of the above described operating cycle leads to deposition of a number of cord sections circumferentially disposed in side by side relationship until the whole circumferential extension of the toroidal support has been covered.
Necessarily, the toroidal support is previously coated with a layer of raw rubber having a dual function, i.e. that of adhering to the cords deposited thereon so as to conveniently hold them in a fixed positioning, and that of constituting an air-proof inner liner in the finished tyre.
Tyres obtained by this production method have a carcass structure in which the carcass ply or plies consist of individual cords each having two side portions axially spaced apart from each other and oriented radially of the rotation axis of the tyre, and a crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tyre beads, the opposite ends of the individual cords forming a carcass ply are arranged, in an alternating sequence, at axially opposite positions relative to an annular anchoring element forming said bead ring, having the form of a crown made up of wire coils radially superposed on each other, as can be seen from patent EP 0 664 231 and patent U.S. Pat. No. 5,702,548. According to the teachings of these documents, a filling body of elastomer material is necessarily interposed between the anchoring annular insert and the threads or wires forming the carcass ply or plies.
In patent FR 384 231, to which referrence is herein made as an example of the most pertinent state of the art, accomplishment of a carcass structure is proposed which involves deposition, on a toroidal support, of a series of rectangular small bands of rubberized fabric circumferentially disposed in side by side relationship and arranged in radial planes relative to the geometric axis of the support drum itself. Deposition of the small bands is carried out in such a manner that the end flaps of two non-consecutive small bands are partly covered with the end flaps of the small band interposed therebetween. The existing spaces between the end flaps covered with the small bands are filled with trapezoidal inserts applied to the end flaps of the small band placed in an overlapping position thereon. Deposition of the small bands is carried out in different superposed layers, the number of which is correlated with the thickness to be given to the carcass structure. The presence of said trapezoidal inserts gives rise to thickening of the carcass structure at the bead regions so that thickness at the bead rings is twice that at the crown.
In U.S. Pat. No. 4,248,287 a method is disclosed according to which formation of the carcass structure contemplates the step of laying down on a toroidal drum, a plurality of layers each formed of radial strips made up of rubberized wires and circumferentially disposed in side by side relationship. When deposition has been completed, two bead rings are applied to the bead region and around said rings the end flaps of the carcass layers formed of the radial strips are turned back.
The Applicant has found that important advantages can be achieved both in terms of simplification of the production processes and in terms of improvement of the behavioural features of the tyre, if the carcass ply or plies are made by conveniently laying down on a rigid toroidal support, strip-like sections each comprising a plurality of cords parallel to each other, incorporated into an elastomer layer.
In this connection, the Applicant has already developed several different manufacturing methods being the object of respective European patent applications.
For instance, in the European patent application Nos. 97830731.2 and 97830733.8 a manufacturing method and a tyre are respectively disclosed in which the carcass structure is accomplished by making a first and a second carcass plies each obtained by strip-like sections sequentially laid down in mutual circumferential side by side relationship.
Tyres obtained in accordance with that which is described in the above patent applications have the end portions of the strip-like sections belonging to the first and second carcass plies disposed on opposite sides respectively, relative to the annular reinforcing structures of the beads.
This expedient, in combination with the respectively crossed orientation of the strip-like sections belonging to one and the other plies, offers important advantages in terms of structural strength of the tyre close to the beads and sidewalls.
In the European patent application No. 98830472.1, in the name of the sane Applicant as well, accomplishment of a carcass ply is proposed which involves deposition of a first and a second series of strip-like sections in an alternating sequence, in which the sections belonging to the first and second series terminate at respectively opposite sides relative to the reinforcing structures at the beads.
Thus important advantages can be achieved in terms of structural strength at the tyre beads and sidewalls, even in the presence of a single carcass ply.
In accordance with the present invention, it has been found that the above mentioned new manufacturing concepts developed by the Applicant and all the advantageous aspects correlated therewith can be also advantageously utilized in the field of tyre manufacture for motorcycles and the like, if particular construction expedients are adopted in making the reinforcing structures at the beads.
In more detail, the Applicant has become aware of the fact that by the construction methodology they have developed, it is surprisingly possible to obtain satisfactory qualities of structural strength at the beads even with use of reinforcing structures comprising a single annular insert formed of at least one elongated element wound up into radially superposed coils. The Applicant has also become aware of the fact that a carcass structure having such annular reinforcing structures at the beads is particularly suitable for manufacturing tyres for motorcycles utilizing the manufacturing methods providing accomplishment of the carcass ply by deposition of strip-like sections on a rigid toroidal support.
In more detail, the invention relates to a method of manufacturing a carcass structure for motor-vehicle tyres, characterized in that accomplishment of each annular reinforcing structure comprises the steps of: laying down at least one elongated element in concentric coils so as to form an annular anchoring insert substantially in the form of a crown, forming at least one filling body of raw elastomer material, joining the filling body to said annular anchoring insert.
In a preferential embodiment, accomplishment of said carcass ply comprises the steps of: laying down on the toroidal support, a first series of said strip-like sections circumferentially distributed with a circumferential pitch corresponding to a multiple of the width of the strip-like sections; applying said annular reinforcing structures against end flaps of said strip-like sections belonging to the first series; laying down on the toroidal support at least one second series of said strip-like sections each extending in a U-shaped conformation around the cross-section outline of the toroidal support, between two consecutive sections of the first series to define said carcass ply therewith, each of the sections of the second series having end flaps overlapping the respective annular reinforcing structures at an axially opposite position relative to the end flaps of the sections of the first series.
It is also preferably provided that the crown portions of the strip-like sections should be consecutively disposed in side by side relationship along the circumferential extension of the toroidal support.
It may be also provided that the side portions of each strip-like section belonging to the first series should be each partly covered with a side portion of at least one circumferentially consecutive section belonging to the second series, at a stretch included between a radially outer edge of the annular reinforcing structure and a transition region between said side portions and crown portions.
In more detail, covering of the side portions of each strip-like section belonging to the first series progressively decreases starting from a maximum value close to the outer circumferential edge of each annular reinforcing structure until a zero value at the transition regions between said side portions and crown portions.
Preferably the side portions of said strip-like sections are caused to radially converge in the direction of the geometric rotation axis of the toroidal support.
Also provided may be at least one operating step aiming at defining regions of increased width close to the inner circumferential edges of the carcass structure.
If, as provided in a preferential embodiment, preparation of said strip-like sections takes place by cutting actions sequentially carried out on at least one continuous strip-like element incorporating said thread-like elements into said layer of raw elastomer material, the step of defining regions of increased width can be advantageously carried out on the continuous strip-like element before execution of the cutting action.
In accordance with a further aspect of the invention, said elongated element is preferably laid down directly against the carcass ply, and more particularly against the end flaps of the strip-like sections belonging to the first series, so as to form the annular anchoring insert directly in contact with the strip-like sections themselves.
The filling body, preferably located at a radially outer position relative to said annular anchoring insert, can be, in turn, formed by laying down a continuous strip of elastomer material directly against the annular insert itself, previously applied to the end flaps of the strip-like sections belonging to the first series.
It is a further object of the invention a carcass structure for tyres of vehicle wheels characterized in that it further comprises a pair of annular reinforcing structures engaged at an area close to respective inner circumferential edges of the carcass ply and each comprising: an annular anchoring insert substantially in the form of a crown disposed coaxially with the carcass structure and adjacent to an inner circumferential edge of the carcass ply, said annular insert being formed of at least one elongated element extending in concentric coils; a filling body of raw elastomer material joined to said annular anchoring insert.
More particularly, said carcass ply is preferably provided to comprise a first and a second series of strip-like sections disposed in a mutually alternating sequence along the circumferential extension of the carcass structure, each of said annular reinforcing structures having an axially inner side turned towards end flaps of the sections belonging to the first series and a axially outer side turned towards end flaps of the sections belonging to the second series.
Preferably, the crown portions belonging to the sections of the first and second series respectively are disposed in mutual side by side relationship along the circumferential extension of the carcass structure.
The side portions of each strip-like section belonging to the first series may be provided to be each partly covered with a side portion of at least one adjacent strip-like section belonging to the second series, at a stretch included between a radially outer edge of the annular reinforcing structure and a transition region between said side portions and crown portions.
In more detail, covering of the side portions of each strip-like section belonging to the first series progressively decreases starting from a maximum value at an area close to the outer circumferential edge of each annular reinforcing structure until a zero value at the transition regions between said side portions and crown portions.
Advantageously, the side portions of said strip-like sections radially converge in the direction of the geometric rotation axis of the carcass structure.
The individual strip-like sections belonging to one of said first and second series respectively are advantageously disposed with a circumferential distribution pitch corresponding to a multiple of the width of the strip-like sections themselves.
Each strip-like section may be provided to have regions of increased width at an area close to the inner circumferential edges of the carcass structure.
In this case, the thread-like elements included in each strip-like section are mutually spaced apart at said regions of increased width.
Preferably, each of said strip-like sections has a width included between 3 mm and 15 mm, and comprises three to eight thread-like elements.
In particular, said thread-like elements are disposed in the respective strip-like sections according to a mutual distance between centres which is not lower than 1.5 times the diameter of said thread-like elements.
According to a further aspect of the invention, said annular anchoring insert has a single series of radially-superposed concentric coils.
It is also preferably provided that said filling body should radially extend from said annular anchoring insert, tapering away therefrom.
In more detail, the ratio between the radial extension of the annular anchoring insert and said filling body can be advantageously included between 0.5 and 2.5.
Conveniently, said filling body of elastomer material has a hardness included between 48xc2x0 and 55xc2x0 Shore D at 23xc2x0 C.